Rough-vacuum pump of dry type

ABSTRACT

A rough-vacuum pump of dry type includes a duct formed in the stator and of which an inlet orifice is intended to be connected to at least one source of purge gas and of which at least one outlet orifice communicates with the last pumping stage communicating with the delivery and a device for injecting a heated purge gas including a heating device to at least partly heat the purge gas injected into the duct to a temperature higher than 40° C.

The present invention relates to a rough-vacuum pump of dry type such as a “roots” or “claw” or screw-type pump.

Rough-vacuum pumps of dry type comprise several pumping stages in series through which a gas that is to be pumped circulates between an intake and a delivery. Among known rough-vacuum pumps, a distinction is made between those having rotary lobes, also known as “roots” pumps, or those having claws, also known as “claw” pumps, or else those of the screw-type. These vacuum pumps are said to be “dry” because, in operation, the rotors rotate inside a stator with no mechanical contact between one another or with the stator, which means that it is possible not to use oil in the pumping stages.

Certain rough-vacuum pumps are used in processes that employ chemicals that generate solid by-products, for example in the form of powder, paste or pieces. This is the case for example with certain processes used in the manufacture of semiconductors, photovoltaic screens, flat screens or LEDs. These solid or condensable by-products may be drawn into the vacuum pump and impair its operation, notably by impeding the rotation of the rotors or, in the worst of cases, even completely preventing them from turning. Certain applications also release by-products or gases that are potentially explosive and/or flammable, such as H₂, SiH₄, TEOS.

In order to limit these risks, the gases pumped are generally diluted with a purge gas injected into the vacuum pump. Nitrogen is generally injected through injection nozzles distributed along the vacuum pump, at each pumping stage. It may sometimes be necessary also to purge the pipe situated downstream of the vacuum pump by injecting an additional purge gas, sometimes heated, to prevent these gases from condensing at the outlet of the vacuum pump, in the pipes or the gas treatment systems (also referred to as abatement systems).

This injection of heated additional purge gas makes it possible to dilute the gases effectively but nevertheless represents a significant cost.

Furthermore, this additional purge is of no benefit to the elements situated upstream of the point of injection of the purge gas and, notably, is of no benefit to the vacuum pump.

It is an object of the present invention to improve the effectiveness of the purging of the vacuum pump. It is another object of the present invention to reduce the costs associated with the purging of the pumped gases.

To this end, one subject of the invention is a rough-vacuum pump of dry type, comprising:

-   -   a stator comprising at least two pumping stages mounted in         series between an intake and a delivery of the vacuum pump,     -   two rotors arranged in the at least two pumping stages, the         rotors being borne by shafts configured to rotate synchronously         in opposite directions in order to entrain a gas that is to be         pumped between the intake and the delivery, characterized in         that the vacuum pump further comprises:     -   a duct formed in the stator and of which an inlet orifice is         intended to be connected to at least one source of purge gas and         of which at least one outlet orifice communicates with the last         pumping stage communicating with the delivery, and     -   a device for injecting a heated purge gas comprising a heating         device configured to at least partly heat the purge gas injected         into the duct to a temperature higher than 40° C.

Heating part of the flow of purge gas injected into the delivery pumping stage makes it possible to increase the flow of injected purge gas significantly without altering the thermal equilibrium of the vacuum pump. The purge gas injected in the delivery pumping stage makes it possible effectively to dilute the pumped gases, notably the potentially explosive and/or flammable gases such as H₂, SiH₄, TEOS or the condensable gases such as the reaction by-products.

The dilution performed at the delivery pumping stage makes it possible to dilute the gases directly in the vacuum pump, as close as possible to the delivery, eliminating cold or inadequately diluted zones between the vacuum pump and a purge gas injection point of the prior art downstream of the vacuum pump. This avoids the pumped gases condensing in the pipes situated downstream of the vacuum pump or in the gas treatment systems and in the vacuum pump.

The life of the vacuum pump can be extended.

In addition, the purge gas injected directly into the vacuum pump is hotter when it reaches the pipes situated downstream than if it had been heated with the same energy and injected directly into these downstream pipes. The electrical power consumption for the heating of the purge gas can therefore be reduced in comparison with an external heating solution, downstream of the vacuum pump.

According to one exemplary embodiment, the outlet orifice of the duct communicates with at least one shaft passage of the stator communicating with the last pumping stage via a clearance between the shaft and the shaft passage. The purge gas is thus diffused in the pumping stage, around the at least one shaft.

The duct communicates, for example, with the two shaft passages of the pumping stage.

The vacuum pump may further comprise at least one sealing device interposed between a lubricated bearing of the vacuum pump and the pumping stage to seal a shaft passage. According to one exemplary embodiment, the duct communicates with the shaft passage between the sealing device and the pumping stage. The purge gas thus injected before the sealing device makes it possible to form a barrier between the pumped gases and the sealing device, making it possible to extend the life of the latter.

According to one exemplary embodiment, the duct has a linear portion connected to the inlet orifice, the linear portion being interposed between the two shafts and communicating with two lune-shaped cavities, each cavity communicating with a respective shaft passage. The purge gas is thus diffused in the pumping stage, around the shafts, by the lune-shaped cavities, before the sealing devices.

The device for injecting a heated purge gas may further comprise:

-   -   a temperature probe in fluidic communication with the purge gas         heated by the heating device, and     -   a controller connected to a power supply of the heating device         and to the temperature probe, to control the temperature of the         purge gas.

The power supply can therefore be controlled to a given temperature setpoint, according to the temperature measurement taken by the temperature probe. The power of the heating of the purge gas can thus automatically adapt according to the flow rate of the purge gas and the demanded temperature setpoint. The controller is also able to cut off the injection of a purge gas by the device for injecting a heated purge gas if it detects a fault with the temperature. This is because it is preferable not to overdilute if the surplus purge gas cannot be heated, because doing so would modify the thermal equilibrium of the vacuum pump, and this would carry the risk of allowing, for example, condensable species to deposit.

The device for injecting a heated purge gas may further comprise a flow rate regulator, for example a manual one, to regulate the flow rate of the purge gas, and a flow rate measurement device configured to measure the flow rate of injected purge gas.

The outlet of the flow rate measurement device may further be connected to the controller which may, for example, cut off the heating power supply if it detects an absence of flow or too low a flow of purge gas.

The device for injecting a heated purge gas may be configured so that the portion of the flow of purge gas that is heated is for example between 10% and 100% of the purge gas injected into the duct. A mixture of, on the one hand, an unheated purge gas (between 0 and 90%) and, on the other hand, a heated purge gas (between 10 and 100%) is thus injected into the duct.

The device for injecting a heated purge gas may be configured so that the flow rate of the heated purge gas is for example greater than 33.8 Pa·m³/s.

The heating device may be configured to heat the purge gas to a temperature lower than 200° C.

According to one exemplary embodiment, the heating device comprises a heating cartridge comprising resistive heating elements arranged in a tube between an inlet and an outlet of the tube in which a purge gas is intended to circulate in order to become heated. The heating cartridge is advantageously positioned as close as possible to the inlet orifice of the duct, for example by means of a thermally insulated pipe.

The vacuum pump may comprise a cooling circuit at least partially incorporated into the stator and surrounding the duct. Thus, the stator is not overheated as a result of the injection of the hot purge gas notably because it can be temperature-controlled by means of the cooling circuit.

The vacuum pump may further comprise a bearing purge duct formed in the stator, intended to be connected to a source of purge gas, and opening into at least one bearing of the vacuum pump.

The vacuum pump may further comprise at least one stage purge duct formed in the stator, intended to be connected to a source of purge gas, and opening into an inter-stage canal connecting the outlet of one pumping stage to an inlet of the following pumping stage.

According to one exemplary embodiment, the vacuum pump comprises a distributor intended to be coupled, on the one hand, to a source of purge gas and, on the other hand, to the bearing purge duct, to the stage purge duct(s), to the duct and to a pipe connected to an outlet of the heating device.

The vacuum pump may further comprise an additional heating device configured to heat at least part of the purge gas distributed by the distributor.

DESCRIPTION OF THE DRAWINGS

Further advantages and features will become apparent from reading the following description of one particular, but nonlimiting, embodiment of the invention, and from studying the attached drawings in which:

FIG. 1 depicts a schematic side view of a vertical longitudinal section through elements of a dry vacuum pump according to the invention.

FIG. 2 shows a view in transverse section of the elements of the vacuum pump of FIG. 1, at one pumping stage.

FIG. 3 shows a schematic view from above of a horizontal longitudinal section through the elements of the vacuum pump of FIG. 1.

FIG. 4 shows a view in transverse section of the elements of the vacuum pump of FIG. 1 at a first support of the vacuum pump.

FIG. 5 shows a schematic view of a device for injecting a heated purge gas.

FIG. 6 shows a schematic view in longitudinal section of a heating device of the device for injecting a heated purge gas.

FIG. 7 shows another schematic depiction of the vacuum pump.

In these figures, the elements that are identical bear the same reference numerals.

The following embodiments are examples. Although the description refers to one or more embodiments, that does not necessarily mean that each reference relates to the same embodiment or that the features apply only to one single embodiment. Simple features of various embodiments may also be combined or interchanged to provide other embodiments.

A rough-vacuum pump is defined as being a positive-displacement vacuum pump configured to, using two rotors, draw in, transfer and then deliver the gas that is to be pumped at atmospheric pressure. The rotors are borne by two shafts driven in rotation by a motor of the rough-vacuum pump. The rough-vacuum pump may be started up from atmospheric pressure.

What is meant by “upstream” is an element that is positioned before another with reference to the direction in which the gas that is to be pumped circulates. By contrast, what is meant by “downstream” is an element that is positioned after another in relation to the direction in which the gas that is to be pumped circulates, the element situated upstream in the direction in which the gases are pumped being at a lower pressure than the element situated downstream.

The rough-vacuum pump 1 of dry type comprises a stator 2 comprising at least two pumping stages 3 a-3 e mounted in series between an intake 4 and a delivery 5 and two rotors 6 arranged in the at least two pumping stages 3 a-3 e (FIG. 1).

The stator 2 is defined by all of the static components of the vacuum pump 1, which notably form the chambers of the pumping stages 3 a-3 e.

The rotors 6 are borne by shafts 7 configured to rotate synchronously in opposite directions to entrain a gas that is to be pumped between the intake 4 and the delivery 5. The shafts 7 are driven by a motor 8 of the vacuum pump 1.

The rotors 6 have, for example, lobes with identical profiles, for example of the “roots” type (FIG. 2) or of the “claw” type, or are of the screw type or employ some other similar positive-displacement vacuum pump principle.

The vacuum pump 1 comprises for example five pumping stages 3 a, 3 b, 3 c, 3 d, 3 e, in which a gas that is to be pumped can circulate. Each pumping stage 3 a, 3 b, 3 c, 3 d, 3 e is formed by a chamber accommodating the rotors 6, the chambers comprising a respective inlet and a respective outlet. During rotation, the gas drawn in from the inlet is trapped in the volume generated between the rotors 6 and the stator 2 and is then entrained by the rotors 6 towards the next stage. The successive pumping stages 3 a-3 e are coupled in series one after another by respective inter-stage canals 9 coupling the outlet of the preceding pumping stage 3 a-3 d to the inlet of the following pumping stage 3 b-3 e. The inlet of the first pumping stage 3 a (also referred to as the low-pressure pumping stage) communicates with the intake 4 of the vacuum pump 1. The outlet of the last pumping stage 3 e (also referred to as the high-pressure pumping stage or the delivery pumping stage) communicates with the delivery 5. The pumping stages 3 b-3 c-3 d interposed between the high-pressure pumping stage 3 e and the low-pressure pumping stage 3 a are referred to as intermediate pumping stages.

The vacuum pump 1 is a rough-vacuum pump which can be started up at atmospheric pressure and is configured to deliver the pumped gases at atmospheric pressure.

The motor 8 driving the shafts 7 is, for example, situated at one end of the vacuum pump 1, for example on the side of the last pumping stage 3 e.

The vacuum pump 1 may comprise at least one sump of lubricant 10. The lubricant, such as grease and/or oil, is able notably to lubricate the rolling bearings of the bearings 11 supporting the shafts 7 of the rotors 6 and the gearwheels 12 for synchronizing the shafts 7 (FIG. 3). The vacuum pump 1 for example comprises a lubricant sump 10 arranged between the motor 8 and the first or last pumping stage 3 a or 3 e. The bearings 11 of the shafts 7 may be lubricated with grease at the other end of the vacuum pump 1, or using a second lubricant sump.

At least one sealing device 13 a, 13 b sealing against lubricants may be interposed between a lubricated bearing 11 and a pumping stage 3 e, 3 a to seal a shaft passage 14 a, 14 b. The sealing devices 13 a, 13 b create very low conductance in the shaft passages 14 a, 14 b around the rotary shafts 7, making it possible to greatly limit the passage of lubricating fluids notably from the lubricant sump 10 towards the dry pumping stages 3 a-3 e, while at the same time allowing the shafts 7 to turn. Each sealing device 13 a, 13 b comprises for example at least one seal, such as two seals, surrounding a shaft 7 (FIG. 3). The seal is, for example, a rubbing seal referred to as a lip seal, a labyrinth seal or a labyrinth, or a combination of these embodiments.

As can be seen in FIGS. 3, 4 and 5, the vacuum pump 1 further comprises a device for injecting a heated purge gas 15 and a duct 16.

The duct 16 is formed in the stator 2. An inlet orifice 16 a of the duct 16 is intended to be connected to at least one source of purge gas. The duct 16 has at least one outlet orifice 16 b, 16 c which communicates with the last pumping stage 3 e which communicates with the delivery 5, to inject a purge gas, such as nitrogen, into the delivery pumping stage 3 e.

The outlet orifice of the duct opens from the stator 2 for example directly into the last pumping stage 3 e, in the housing accommodating the rotors 6 (which is not depicted). According to another exemplary embodiment, the outlet orifice 16 b, 16 c of the duct 16 communicates with at least one shaft passage 14 a, 14 b communicating with the last pumping stage 3 e via a clearance between the shaft 7 and the shaft passage 14 a, 14 b of the stator 2. The duct 16 thus passes through the stator 2 to carry the purge gas into the pumping stage 3 e through the at least one shaft passage 14 a, 14 b. The purge gas is thus diffused around the at least one shaft 7 in the pumping stage 3 e.

The duct 16 communicates with the at least one shaft passage 14 a, 14 b between the sealing device 13 a, 13 b and the pumping stage 3 e. The purge gas thus injected before the sealing devices 13 a, 13 b makes it possible to form a barrier between the pumped gases and the sealing devices 13 a, 13 b, making it possible to extend the life of the latter.

More specifically, the duct 16 may be made in a first support 17 (or high-pressure support) of the stator 2 arranged against the high-pressure pumping stage 3 e (FIGS. 1 and 3). The duct 16 communicates for example with the two shaft passages 14 a, 14 b of the pumping stage 3 e (FIG. 4).

According to one exemplary embodiment, the duct 16 is created by machining. It has for example a linear portion 18 connected to the inlet orifice 16 a. The linear portion 18 is perpendicular to the axis of the shafts 7 and interposed between the two shafts 7. The linear portion 18 communicates with two cavities 20 a, 20 b, in this instance lune-shaped. The diameter of the lune is substantially less than that of the shaft passage 14 a, 14 b. The lunes are connected back to back by the linear portion 18. Each cavity 20 a, 20 b communicates with a respective shaft passage 14 a, 14 b in the region of the outlet orifices 16 a, 16 b of the duct 16. The purge gas is thus diffused in the pumping stage 3 e, around the shafts 7, by the lune-shaped cavities 20 a, 20 b, before the sealing devices 13 a, 13 b.

The inlet orifice 16 a of the linear portion 18 opening onto the exterior of the stator 2 is, for example, coupled by at least one pipe 21 a, 21 b, such as a hose, to a coupling 22, for example mounted on a chassis of the vacuum pump 1 and intended to be coupled to a source of purge gas external to the vacuum pump 1.

The device for injecting a heated purge gas 15 comprises a heating device 24 configured to at least partly heat the purge gas injected into the duct 16 to a temperature higher than 40° C. The heating temperature is, for example, lower than 200° C.

A setpoint temperature to which to heat the purge gas of 100° C. is, for example, anticipated.

The portion of the flow of heated purge gas injected into the duct 16 is, for example, between 10% and 100%.

The flow rate of the heated purge gas is, for example, higher than 20 slm (i.e. 33.8 Pa·m³/s) and, for example, lower than 200 slm (i.e. 338 Pa·m³/s), such as 120 slm (i.e. 202.7 Pa·m³/s).

A mixture of heated purge gas and of unheated purge gas may thus be injected into the duct 16.

For example, for a purge-gas flow rate of 120 slm and a heated portion of the flow of purge gas of 50%, the flow rate of the heated purge gas is 60 slm (i.e. 101.35 Pa·m³/s) and the flow rate of the unheated purge gas is also 60 slm.

It is also possible to heat all of the purge gas which is injected into the duct 16 (100%).

Heating part of the flow of purge gas injected into the delivery pumping stage 3 e makes it possible to increase the flow of purge gas injected significantly without modifying the thermal equilibrium of the vacuum pump 1. The mixture of the potentially unheated purge gas and of the heated purge gas, which is injected at the at least one shaft passage 14 a, 14 b, makes it possible to increase the flow of purge gas significantly in order to effectively dilute the pumped gases, notably the potentially explosive and/or flammable gases such as H₂, SiH₄, TEOS or the condensable gases such as the reaction by-products.

The dilution achieved at the delivery pumping stage 3 e makes it possible to dilute the gases directly in the vacuum pump 1, as close as possible to the delivery 5. Injecting a large quantity of purge gas directly into the last pumping stage 3 e makes it possible to eliminate cold or inadequately diluted zones between the vacuum pump 1 and a purge gas injection point of the prior art, downstream of the vacuum pump. This prevents the pumped gases from condensing in the pipes situated downstream of the vacuum pump 1 or in the gas treatment systems and in the vacuum pump 1.

The life of the vacuum pump can be extended.

In addition, the purge gas injected directly into the vacuum pump 1 is hotter when it reaches the pipes situated downstream than if it had been heated with the same energy and injected directly into these pipes downstream. The electrical power consumption for the heating of the purge gas can therefore be reduced in comparison with an external heating solution, downstream of the vacuum pump 1.

According to one exemplary embodiment, the vacuum pump 1 comprises a cooling circuit 25 for cooling the stator (FIG. 4). The cooling circuit 25 comprises for example a hydraulic circuit allowing water to circulate, for example at ambient temperature. The cooling circuit 25 is, for example, at least partially integrated into the stator 2, for example into the first support 17 in which the duct 16 is formed. It has for example the shape of a U surrounding the pipe 16. Thus, the stator 2 is not overheated as a result of the injection of the hot purge gas notably because it can be temperature-controlled using the cooling circuit 25.

According to one exemplary embodiment that can be seen in FIG. 6, the heating device 24 comprises a heating cartridge comprising resistive heating elements 26 arranged in a tube 27 between an inlet 27 a and an outlet 27 b of the tube 27 in which a purge gas is intended to circulate in order to become heated. The heating cartridge is advantageously positioned as close as possible to the inlet orifice 16 a of the duct 16.

The pipe 21 b connecting the outlet 27 b of the tube 27 of the heating cartridge to the duct 16 of the vacuum pump 1 is, for example, thermally insulated.

According to one exemplary embodiment, the device for injecting a heated purge gas 15 further comprises a temperature probe 28 in fluidic communication with the purge gas heated by the heating device 24 and a controller 30, such as an electronic board, connected to a power supply of the heating device 24 and to the temperature probe 28. The controller 30 is configured to control the temperature to which the purge gas is heated.

The temperature probe 28, such as a thermocouple, is arranged for example at one end of the tube 27 on the side of the outlet 27 b and the power supply to the resistive heating elements 26, for example the supply of current, passes via the other end of the tube 27, on the side of the inlet 27 a, through a sealed passage 31.

The power supply can thus be controlled to a given temperature setpoint, according to the temperature measurement taken by the temperature probe 28. The power for the heating of the purge gas in the heating cartridge can thus adapt automatically according to the flow rate of the purge gas and according to the demanded temperature setpoint.

The controller 30 is also able to cut off the injection of a purge gas by the device for injecting a heated purge gas 15 if it detects a fault with the temperature. This is because it is preferable not to overdilute if the surplus purge gas cannot be heated because doing so would modify the thermal equilibrium of the vacuum pump 1, and this would risk for example allowing condensable species to deposit.

The device for injecting a heated purge gas 15 may further comprise a flow rate regulator 32, for example a manual one, to regulate the flow rate of the purge gas, and a flow rate measurement device 33 configured to measure the flow rate of injected purge gas.

The outlet of the flow rate measurement device 33 may also be connected to the controller 30 which may for example cut off the heating power supply if it detects an absence of flow or a purge flow that is too low.

The flow rate regulator 32 and the flow rate measurement device 33 are, for example, arranged upstream of the heating device 24 in the direction in which the purge gas flows from the source of purge gas towards the heating device 24 (see the arrows in FIG. 5). An isolation valve 34 may be arranged upstream of the heating device 24, for example between the flow rate regulator 32 and the flow rate measurement device 33. The device for injecting a heated purge gas 15 is, for example, mounted on the chassis of the vacuum pump 1 supporting the stator 2.

The vacuum pump 1 may also comprise a bearing purge duct 35 formed in the stator 2 and opening into at least one bearing 11 of the vacuum pump 1 and/or at least one stage purge duct 36 formed in the stator 2 and opening into an inter-stage canal 9 coupling the outlet of one pumping stage 3 a-3 d to an inlet of the following pumping stage 3 b-3 e, to distribute a purge gas through all or part of the inter-stage canals 9 (FIG. 1).

The bearing purge duct 35 passes through the stator 2 to convey the purge gas to the at least one bearing 11 accommodating the rolling bearings.

More specifically, for example, the bearing purge duct 35 is created in a second support 37 (or low-pressure support) of the stator 2 in which support the bearings 11 are mounted (FIGS. 1 and 3). The second support 37 is arranged against the low-pressure pumping stage 3 a.

The bearing purge duct 35 communicates for example with the two bearings 11. According to one exemplary embodiment, the bearing purge duct 35 has a first linear portion, perpendicular to the axis of the shafts 7 and interposed between the two shafts 7. The first linear portion communicates with a second linear portion the ends of which communicate with a respective bearing 11 (FIG. 3).

The stage purge duct 36 passes through the stator 2 to convey the purge gas to at least one inter-stage canal 9. The vacuum pump 1 comprises for example three stage purge ducts 36 opening into a respective inter-stage canal 9, for example near the outlet of the pumping stage 3 b-3 d.

The bearing purge duct 35, the stage purge ducts 36, the duct 16 and the pipe 21 b connected to the outlet 27 b of the heating device 24 are intended to be connected to a source of purge gas, external to the vacuum pump 1, via, for example, a distributor 38 (FIG. 7).

According to one exemplary embodiment, the distributor comprises a common trunk 38 a connected to a first and a second branch 38 b, 38 c. The first branch 38 b connects the common trunk 38 a to the bearing purge duct 35 created in the second support 37 of the stator 2. The second branch 38 c is connected by ducts 38 d, 38 e to the stage purge ducts 36 and to the duct 16.

The inlet of the common trunk 38 a is intended to be coupled to a source of purge gas, such as nitrogen. A first isolation valve and an expansion valve are arranged for example on the common trunk 38 a, a second isolation valve may be arranged on the first branch 38 b and a third isolation valve may be arranged on the second branch 38 c. It is thus possible to carry out a purge suited to the nature of the pumped gases. For example, only the bearings 11 are purged in the case where it is non-condensable and/or non-flammable gases being pumped.

The first branch 38 b and the three ducts 38 d connected to the stage purge ducts 36 are, for example, provided with injection nozzles or spray injectors making it possible to define a flow rate of the purge flow in the bearing purge duct 35 and in each stage purge duct 36. The duct 38 e connected to the duct 16 may also comprise an injection nozzle or spray injector, arranged upstream of the intersection of the duct 38 e with the pipe 21 b, connected to the outlet of the heating device 24 in the direction of circulation of the purge gas.

The purge gas injected into the distributor 38 via the duct 38 e, the bearing purge duct 35 and the stage purge ducts 36 is able not to be heated. However, the vacuum pump 1 may comprise an additional heating device 39 configured to heat at least partially the purge gas distributed by the distributor 38, in the first and/or the second branch 38 b, 38 c and/or in the duct 38 e connected to the duct 16 and to the pipe 21 b. The additional heating device 39 is, for example, arranged on the second branch 38 c connected to the ducts 38 d, 38 e.

This heating may be independent of the heating of the purge gas by the heating device 24. The flow rate and/or the temperature can be controlled independently for each pumping stage 3 b-3 e and for the bearings 11 of the second support 37, or at least independently of the flow rate and of the temperature of the purge gas injected by means of the device for injecting a heated purge gas 15. 

1.-15. (canceled)
 16. A rough vacuum pump of dry type, comprising: a stator comprising at least two pumping stages mounted in series between an intake and a delivery of the vacuum pump; two rotors arranged in the at least two pumping stages, the rotors being borne by shafts configured to rotate synchronously in opposite directions in order to entrain a gas that is to be pumped between the intake and the delivery; a duct formed in the stator and of which an inlet orifice is configured to be connected to at least one source of purge gas and of which at least one outlet orifice communicates with the last pumping stage communicating with the delivery; and a device to inject a heated purge gas comprising a heating device configured to at least partly heat the purge gas injected into the duct to a temperature higher than 40° C.
 17. The vacuum pump according to claim 16, wherein the outlet orifice of the duct communicates with at least one shaft passage of the stator communicating with the last pumping stage via a clearance between the shaft and the shaft passage.
 18. The vacuum pump according to claim 17, wherein the duct communicates with the two shaft passages of the pumping stage.
 19. The vacuum pump according to claim 18, wherein the duct has a linear portion connected to the inlet orifice, the linear portion being interposed between the two shafts and communicating with two lune shaped cavities, each cavity communicating with a respective shaft passage.
 20. The vacuum pump according to claim 17, further comprising at least one sealing device interposed between a lubricated bearing of the vacuum pump and the pumping stage to seal the shaft passage, wherein the duct communicates with the shaft passage between the sealing device and the pumping stage.
 21. The vacuum pump according to claim 16, wherein the device to inject a heated purge gas further comprises: a temperature probe in fluidic communication with the purge gas heated by the heating device, and a controller connected to a power supply of the heating device and to the temperature probe, to control the temperature of the purge gas.
 22. The vacuum pump according to claim 16, wherein the heating device comprises a heating cartridge comprising resistive heating elements arranged in a tube between an inlet and an outlet of the tube in which a purge gas is configured to circulate in order to become heated.
 23. The vacuum pump according to claim 16, wherein the device to inject a heated purge gas is configured so that the portion of the flow of purge gas that is heated is between 10% and 100% of the purge gas injected into the duct.
 24. The vacuum pump according to claim 16, wherein the device to inject a heated purge gas is configured so that the flow rate of the heated purge gas is greater than 33.8 Pa·m³/s.
 25. The vacuum pump according to claim 16, wherein the heating device is configured to heat the purge gas to a temperature lower than 200° C.
 26. The vacuum pump according to claim 16, further comprising a cooling circuit at least partially incorporated into the stator and surrounding the duct.
 27. The vacuum pump according to claim 16, further comprising a bearing purge duct formed in the stator, configured to be connected to a source of purge gas, and opening into at least one bearing of the vacuum pump.
 28. The vacuum pump according to claim 16, further comprising at least one stage purge duct formed in the stator, configured to be connected to a source of purge gas, and opening into an inter stage canal connecting the outlet of one pumping stage to an inlet of the following pumping stage.
 29. The vacuum pump according to claim 16, further comprising a distributor configured to be coupled to a source of purge gas and to the bearing purge duct, to the stage purge duct(s), to the duct and to a pipe connected to an outlet of the heating device.
 30. The vacuum pump according to claim 29, further comprising an additional heating device configured to heat at least part of the purge gas distributed by the distributor. 